1. Field of Invention
This invention relates to reduction of energization transients in three phase power systems and more particularly to a method of reducing energization transients in a Y-connected load and to an energization transient limited three phase AC power system apparatus.
2. Description of Related Art
Transformers, reactors and capacitors are common equipment in utility power systems. When such equipment is energized by a power system it normally produces energization transients such as voltage and/or current surges. These transients are often the result of inrush currents into the load being energized. Energization transients are often reflected back into the power system which can disrupt the operation of power quality sensitive electrical loads such as computers and medical equipment connected to the system. Power quality sensitive electronics are in increasing use and thus control of energization transients has become an important concern to the power industry.
Over the past twenty years, two main methods have evolved to limit inrush currents as a way of reducing energization transients. A first method involves pre-insertion of damping resistors or inductors. This involves insertion of an impedance such as a resistor and/or inductor in series with each leg of the load. Each resistor and/or inductor is then bypassed after the leg has been energized and has reached a steady state condition. The use of a resistor tends to increase the damping of the overall circuit, resulting in faster disappearance of transients. The main disadvantage of this method is the need for a separate bypass circuit breaker and resistor for each phase. Systems of this type are described in U.S. Pat. Nos. 6,018,473, 6,218,652, 4,695,918, for example.
A second method involves precisely controlling the closing instant of the switches or breakers connecting the load to the power system such that each breaker is closed at precisely the instant when the system voltage or current, depending on the load, in the corresponding phase is zero. If the load is capacitive, the breaker associated with each phase is closed when the voltage of the corresponding phase is zero. If the load is inductive, the breaker is closed when the voltage is maximum and current is zero. With the advancement of microprocessor technology, this method has become technically feasible and has gained acceptance in industry. This method however requires specially designed circuit breakers that have consistent mechanical operation characteristics over a wide range of installation and temperature conditions. The tolerance for error on closing time is very small (less than 2 to 5 milliseconds). The reliability of such a scheme and the sophistication involved makes it unattractive for general applications. This technology and its variations are described in U.S. Pat. Nos. 5,838,077, 6,523,654, for example.
A variation of the first method above, for use with transformer loads involves the connection of a resistor between the transformer neutral and ground. This scheme has been described in B. Holmgrem, R. S. Jenkins and J. Rinbrugent, “Transformer inrush current,” CIGRE Proc. Of the 22nd session, vol. 1, 12-03, pp. 1-13, 1968 and R. Yacamini and A. Abu-Nasser, “The calculation of inrush current in three-phase transformers,” IEE Proc.-Electr. Power Appl., vol. 133, no. 1, pp. 31-40, January 1986. Both papers describe concurrent energization of all three legs and both concluded that the neutral resistor has no effect on limiting inrush currents.
Another attempt to simplify the first method is described in U.S. Pat. No. 6,337,802 and involves the use of one series resistor switched into different phases of the load so that the effect of three resistors is at least partially realized. This method appears to be suited for low or medium voltage converter applications and could be more expensive than the basic series resistor method if applied to high voltage power systems as it requires several additional switches.
The present invention addresses the above problems.